Interpretive Summary: The genetic diversity of cucumber market types (fresh market and processing) and exotic germplasm (i.e., plant introductions) has been well documented, and found to be extremely narrow (3-8% among cultivars). This lack of genetic diversity has been an impediment to the genetic improvement of cucumber in several commercially important market classes. Recently, Chinese and USDA, ARS researchers have successfully made an inter-specific cross between commercial cucumber and a wild type cucumber (C. hystrix) to produce a plant product (hybrid) that has potential to increase the genetic diversity of commercial cucumber and thereby allow for more opportunities for improvement of cucumber. A study was designed to further refine this hybrid by complex matings with another commercial cucumber type followed by improvement using biotechnologies (i.e., marker-assisted selection = MAS). The resulting progeny were evaluated in field trials to determine their potential for use in modern plant breeding programs. The results indicated that the resulting progeny were very diverse genetically (as defined by MAS) and had yield and quality characteristics that would allow their successful inclusion into private and public plant breeding programs for cucumber improvement. The resulting progeny will provide a new source of plant material currently not available in commercial cucumber to improve the yield and quality of this crop species. The resulting products (cultivars) will make the U.S. grower more competitive in the global market place and provide managerial flexibility that does not now exist in production of this crop.

Technical Abstract:
The genetic base of commercial cucumber (Cucumis sativus L.) is extremely narrow (about 3 to 8% polymorphism). Wide-based crosses within C. sativus [i.e., var. sativus x var. hardwickii (R.) Alef.] and interspecific hybridization attempts prior to 1995 have not substantially increased genetic diversity for plant improvement. However, in 1995, an amphidiploid (Cucumis hytivus Chen and Kirkbride) was derived from a C. sativus x Cucumis hystrix Chakr. mating. A derivative of this amphidiploid was used herein to broaden the genetic base of cucumber through backcross introgression [(C. sativus x C. hytivus) x C. sativus]. Initially, the combining ability of eight genetically diverse lines was investigated for days to anthesis (DA), sex expression (SEX), lateral branch number (LBN), fruit per plant (FP), fruit lenth-by-diameter ratio (L:D), and salt-processing ability [i.e., processed fruit color (exterior and interior), shape, and seed cavity characteristics]. Based on the combiing ability, inbred backcross lines (BC2S3; IBL) were developed from an original gynoecious determinate line WI 7023A (C. sativus; recurrent parent) x monoecious indeterminate line WI 7012A (C. sativus x C. hytivus derived) mating, where 30 of 392 (8%) BC1 progeny were selected based on their diversity at 16 mapped marker loci. These progeny were used to develop BC2 progeny, which were then self-pollinated without further selection to produce 94 IBL. These IBL were genotyped and evaluated in the open-field in two plantings in 2008 for DA, SEX, LBN, leaf size, FP, and L:D. The genetic distance (GD) between parental lines was 0.85, and the GD among IBL ranged between 0.16 and 0.75. Multivariate analyses indicated that IBL differed from parental lines and possessed considerable morphological and genotypic diversity that could be used to broaden the genetic base of commercial U.S. processing cucumber.